Which muscles act as primary agonists for medial rotation of the shoulder?

• A. Infraspinatus, Teres Minor, Deltoid (Posterior)
• B. Deltoid (Anterior), Pectoralis Major, Subscapularis, Latissimus Dorsi
• C. Pectoralis Minor, Serratus Anterior
• D. Trapezius, Levator Scapulae

Answer: (B)

Medial rotation of the shoulder is produced by muscles that pull the humerus toward the midline, especially when the arm is flexed or otherwise positioned to emphasize internal rotation. The main internal rotators at the glenohumeral joint are the subscapularis, pectoralis major, latissimus dorsi, and teres major. Among these, subscapularis is the primary internal rotator and a strong stabilizer of the front of the joint. The anterior deltoid can also contribute to internal rotation, particularly when the arm is flexed, acting as a synergist with the main internal rotators.

The other muscles listed don’t primarily rotate the humerus inward: infraspinatus, teres minor, and posterior deltoid are external rotators; pectoralis minor and serratus anterior mainly move the scapula; trapezius and levator scapulae mainly move or position the scapula and neck rather than medially rotating the humerus.

So the combination of anterior deltoid, pectoralis major, subscapularis, and latissimus dorsi best represents the muscles that act to medially rotate the shoulder.